With the growing need to downsize devices, scientists have been attempting to construct molecular scale devices, which perform applications similar to those of their macroscopic counterparts. To this end, we would like to design systems (either molecular or supramolecular) that promote efficient and controllable charge or energy transfer. Cyclic peptides offer an excellent scaffold for the construction of such systems, as they readily self-assemble into peptide nanotubes. Through selective chromophore substitution on these cyclic-peptides, long range energy and/or charge transfer can occur, which allows the nanotube system to function like a molecular scale wire. To date, we have focused on the novel synthesis of cyclic peptides containing pyrene. While these systems will likely undergo long-range transfer, they will also serve as a diagnostic for the nature of cyclic peptide self-assembly, due to the unique photochemistry of pyrene.
Since 1959, the number of devices on an integrated circuit chip has increased from 1 to 10 million. As a result there is an increasing demand for nanoscale materials with new electronic, chemical and physical properties. Nanotubes are of special interest for different utilities, for example, containment of nanowires, optical and electronic devices, catalytic media, therapeutic agents, trans-membrane channels and drug delivery vehicles. Carbon graphite nanotubes, boron nitride nanotubes, zeolites, and crown ether based tubes are well-known nanostructures.
In 1993, self-assembling cyclic D, L-peptides were observed as hollow tubular structures by electron microscopy, electron diffraction, FT-W, and molecular modeling. Cyclic peptides with an even number of alternating D-, L-amino acids preferentially adopt flat disk like conformations in which all backbone amide functionalities lie approximately perpendicular to the plain of the structure. In this confirmation, subunits can stack in an anti-parallel fashion and participate in backbone—backbone intermolecular hydrogen bonding to produce a contiguous n-sheet structure. Moreover, because of the D- and L-amino acids sequence, peptide side chains must necessarily lie on the outside of the ensemble thereby resulting in the desired hollow tubular structure. When protonated, cyclic polypeptides crystallize into tubular structures hundreds of nanometer long, with internal diameters of 7-8 Å. We are making cyclic peptides with different amino acids that have two different chromophore groups on one cycle. Nowadays, we are focused on the difference between the cyclic peptides that make dimer stacking, non-stacking and stacking in terms of energy transfer between the chromophores.